The present invention relates to a Coriolis flowmeter and, more particularly, to the construction of a double-conduit type or counterbalanced type Coriolis flowmeter.
When a measuring conduit, supported at both ends on a supporting means, is driven with alternate oscillation at its center portion in a direction perpendicular to its axis, a phase difference is produced between the supporting points and the center portion of the measuring conduit. This phase difference is produced by the action of a Coriolis force and has a value proportional to the driving frequency and mass flow rate. A straight conduit type Coriolis flowmeter which measures mass flow by detecting the phase difference of its straight measuring conduit is well known.
A straight conduit type Coriolis flowmeter has a most simple configuration and may be formed smaller in the direction perpendicular to the fluid's direction of flow but the straight measuring conduit, supported at both ends, has a high amount of rigidity in the direction perpendicular to its axis and has low sensitivity with a low S/N ratio. To attain high sensitivity, the measuring conduit must have thin walls and be elongated enough in the flow's direction. This means that a straight conduit is not always advantageous in the construction of the flowmeter. In addition, thus designed flowmeters are easily effected by external vibrations due to decreased resonance frequency. Furthermore, the natural frequency of the measuring conduit shall be changed by the deformation due to the pressure deviation of the fluid in the measuring conduit. The Japanese publication of the unexamined application No. 63-158419 discloses a straight conduit type Coriolis flowmeter wherein at least one measuring conduit is mounted within a supporting cylinder and supported at both ends by means of ring diaphragms. The measuring conduit having both ends fixed, may be subjected to mechanical stress produced therein by thermal deformation and thereby its natural frequency changes, resulting in the transmission of the oscillating energy of the conduit to the supporting cylinder and a connecting conduit. The application of the ring diaphragms aims to solve the above-mentioned problem by utilizing its elasticity.
This method is effective for removing noise due to measuring conditions if the straight conduit remains the same size and form but it involves the problem that the ring diaphragms, directly supporting the measuring conduit ends, may show signs of fatigue, after a long period of use, resulting in the decreased reliability of its operation. Furthermore, it is well known that the measuring pipe has a radically enlarged section at the ring diaphragms, whereat fluid cavities appear because of noisy vibrations, resulting in a serious decrease in the stability of the measurement. To sense the Coriolis force with high sensitivity, the conventional straight conduit type flowmeter must have an elongated measuring conduit to decrease the rigidity or it will require an increase in the quantity of flowing fluid. If the measuring conduit is made longer, it developes a lower natural frequency and thereby is easily effected by external vibrations. Increasing the quantity of the fluid results in increasing a loss of pressure in the measuring conduit.
As described above, a conventional Coriolis flowmeter has a driving means to drive a measuring conduit having fixed ends to oscillate at its natural oscillation frequency by which the driving energy can be minimized. For this purpose, a positive feedback loop is composed of a measuring conduit, a driving means for oscillating the measuring conduit, a sensor for sensing the oscillating amplitude of the measuring conduit (This sensor is commonly used for sensing the Coriolis force), and an amplifier circuit for amplifying the sensor's signal and the driving means to oscillate the conduit at a constant oscillating amplitude.
The driving means may be composed of e.g., a core and an electromagnetic coil, to which AC current is supplied to produce an alternating electromagnetic force used to attract and repel the core. This driving means is mounted between a fixed base and the measuring conduit disposed parallel to the fixed base and resting at both ends thereon. To oscillate the measuring conduit more effectively, a counterbalance made in the form of a conduit, a column or a plate having the same natural frequency as the measuring conduit, is used instead of the base, in such a way that the counterbalance is disposed parallel to the measuring conduit and supported at both ends by the supports of the measuring conduit while the driving means is disposed between the measuring conduit and the counterbalance to drive the measuring conduit and the counterbalance like a tuning fork being vibrated.
However, the flowmeter may be used for measuring the flowrate of different kinds of fluids passing through its measuring conduit. The measuring conduit varies in its natural frequency depending on the density of the fluid to be measured. Even when the same kind of fluid is measured, the fluid density varies depending upon its temperature and thereby the natural frequency of the measuring conduit changes. Therefore, there may be a difference between the natural frequency of the measuring conduit and the fixed natural frequency of the counterbalance with no fluid flowing. This means that the measuring conduit cannot effectively be driven.